In a medium saucepan, bring blueberries, sugar, and lemon juice to a boil over medium high heat. Reduce heat and gently boil for 5-10 minutes.
Remove from heat and cool completely.
With an electric mixer, whip cream until stiff peaks form.
To a large bowl, add sweetened condensed milk. Gently in the whipped cream and vanilla flavoring. Add the cooled blueberry mixture and stir just until combined.
Pour mixture into a loaf pan and cover the top with a piece of wax paper. Freeze for at least six hours or overnight before serving. Keeps well for two weeks.